A scroll compressor used in an on-vehicle air conditioner includes a fixed scroll and a rotational scroll. The fixed scroll and the rotational scroll are each a circular end plate with a spiral wrap integrally formed on one of surfaces thereof. The fixed scroll and the rotational scroll are placed facing each other with their wraps being meshed, and the rotational scroll orbits relative to the fixed scroll to decrease the volume of a compression pocket formed between the two wraps while moving the compression pocket radially from outward to inward, thereby performing compression of refrigerant gas.
At actuation of the scroll compressor, reaction force due to the compressed refrigerant gas is applied to the end plate of the rotational scroll and the end plate of the fixed scroll. Thus, the rotational scroll is pressed in a direction in which the rotational scroll becomes separated from the fixed scroll in an axial direction, so that a gap called chip clearance is likely to be generated between a leading end surface (tooth top) of the wrap of each scroll and the other end plate. The refrigerant gas is leaked through the chip clearance, leading to degraded efficiency of the compressor.
For example, PTLs 1 and 2 each disclose a scroll compressor in which a back-pressure chamber is formed adjacent to a back side of the end plate of the rotational scroll with (or without) a thrust plate interposed therebetween, and part of the refrigerant gas compressed in the compression pocket is extracted and supplied to the back-pressure chamber so as to press the rotational scroll toward the fixed scroll so that the leading end surface of each wrap is constantly in contact with the other end plate.
When the back-pressure chamber adjacent to the back side of the end plate of the rotational scroll is formed to press the rotational scroll as described above, a view in an axial direction of a main shaft configured to rotationally drive the rotational scroll indicates that the back-pressure chamber is shaped in a ring around the main shaft. Such a ring-shaped back-pressure chamber has a larger area (width) with a smaller inner diameter and a larger outer diameter, thereby achieving enhanced pressing force on the rotational scroll.